Aqueous latexes of polyvinylidene chloride (PVdC) have been used to form moisture and oxygen barrier coatings for many years. In a typical coating process, the PVdC latex is coated onto paper, film, and other substrates to yield cost-effective packaging constructions with excellent barrier properties. During the application process, the PVdC is first coated onto the substrate and then dried by being exposed to infrared radiation, hot air, or a combination thereof. Often, two layers of PVdC are applied to the substrate to improve the barrier properties of the resulting composite.
The composite is typically wound into a roll for later processing. A major drawback with the presently available PVdC coating processes is that, under certain circumstances, the PVdC tends to adhere to the uncoated side of the substrate. Thus, the composite is prone to blocking when the roll is unwound.
While not wishing to be bound by theory, there are several factors that might contribute to this blocking effect. First, there is often a chemical affinity between the PVdC and the uncoated side of the substrate to which it is applied, so that the PVdC tends to adhere to the substrate when the composite is wound into a roll. Next, any residual moisture resulting from the coating process can cause the layers to adhere to one another. Further, the crystallinity of PVdC, which is a function of the temperature and time, has been shown to impact the degree of blocking. PVdC adherence to the uncoated side of the substrate tends to diminish as the degree of crystallinity increases. However, the minimal application of heat to the PVdC barrier material during the drying process does not sufficiently drive the degree of crystallinity to a level at which no blocking occurs. Additionally, the tension used during the winding process can impact blocking in the composite. The greater the tension, the more tightly wound the layers of the composite are forced against each other. Surface rewinders, which have primary and secondary arms that force the composite roll tightly against a winder drum, result in greater compression in the composite roll. Such compression also contributes to blocking. In some instances, such materials can successfully be wound on surface winders by carefully setting the winding speed, tension, and so forth. However, lack of repeatability and inconsistent product quality continue to present challenges. Center winders with taper tension winding control are more suited to wind rolls of the composite at low tension, but blocking may still occur.
Thus, there remains a need for a method of forming a PVdC coated substrate and a roll of such substrate that is not prone to blocking.